Global climate change influences the frequency and magnitude of hydrological fluctuations, causing devastating events such as floods and drought. These events, generating stress to plants, are the primary cause of crop loss worldwide, causing average yield losses of more than 50% for major crops. Drought is the major factor that globally limits crop productivity, and flooding represents another severe constraint that limits crop growth and productivity. Both high and low extremes in precipitation increasingly limit food, fiber and forest production worldwide (Easterling et al., PNAS. 104:19679 (2007)).
Progress has been made in plant transformation for enhanced abiotic stress tolerance through manipulation of either transcription and/or signaling factors during the last two decades. However, none of these transformed crops has yet been marketed.
Transcription factors (TFs) regulate gene expression in response to environmental stresses. Bioinformatics studies have indicated that Arabidopsis has around 2000 genes encoding TFs (Riechmann et al., Science 290:2105-2110 (2000); and Mitsuda et al., Plant and Cell Phys. 50:1232-1248 (2009)), which are classified into different families according to the structure of their binding domains as follows: NAC, DOF, WRKY, bZIP, ERF/AP2, MYB, Zn-finger and homeodomain (HD). HD is a 60 amino acid DNA-binding domain that is present in TFs across all eukaryotic organisms, and which is encoded by a 180 bp sequence designated as Homeobox (HB). In plants, HD proteins are divided into different families, for example, HD-Zip, PHD finger, Bell, ZF-HD, WOX and KNOX (Chan et al., Biochim. Biophys. Acta 1442:1-19 (1998); Ariel et al., Trends Plant Sci. 12:419-426 (2007)). Members of the HD-Zip family exhibit an association of a HD and a downstream leucine zipper motif (LZ). This association in a single protein is unique to plants (Schena et al., PNAS 89, 3894-3898 (1992)). HD-Zip proteins can be divided into four subfamilies, named I to IV, according to several structural and functional features.